The visual effects of multipath distortion upon analog television signals can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. Macro-ghosts are more common in over-the-air terrestrial broadcasts than in cablecasting. Long-delay multipath effects, or macroghosts, are typically reduced by cancellation schemes.
In the usual case in which the direct signal predominates and the receiver is synchronized to the direct signal, the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. Typically, the range for post-ghosts extends to 40 microseconds displacement from the "principal" signal, with 70% or so of post-ghosts occurring in a sub-range that extends to 10 microseconds displacement.
In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images. Pre-ghosts occurring in off-the-air reception can be displaced as much as 6 microseconds from the "principal" signal, but typically displacements are no more than 2 microseconds.
Multipath signals delayed relatively little with respect to the reference signal do not cause separately discernible copies of the predominant image, but do introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay, close-in or nearby ghosts are commonly caused by unterminated or incorrectly terminated radio-frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by the reflections introduced by having several improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts", and they can accumulate to cause significant distortion. Short-delay multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the video frequency response.
In September 1995 the Advanced Television Systems Committee (ATSC) published a standard for digital high-definition television (HDTV) signals that has been accepted as the defacto standard for terrestrial broadcasting of digital television (DTV) signals in the United States of America. The standard will accommodate the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal. The standard uses vestigial-sideband (VSB) amplitude modulation (AM) to transmit the DTV signals, designed for transmission through 6-Mz-bandwidth ultra-high-frequency (UHF) channels that correspond to channels currently used for analog television transmission.
DTV transmitted by VSB AM during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments or data lines. Each segment of data is preceded by a data segment synchronization (DSS) code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The segments of data are each of 77.3 microsecond duration, and there are 832 symbols per data segment for a symbol rate of about 10.76 million bauds or symbols per second. The initial line of each data field is a data field synchronization (DFS) code group that codes a training signal for channel-equalization and multipath suppression procedures. The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3rate punctured trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal exclusive of pilot carrier, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
Ghosts are a problem in digital television (DTV) transmissions as well as in NTSC analog television transmissions, although the ghosts are not seen as such by the viewer of the image televised by DTV. Instead, the ghosts cause errors in the data-slicing procedures used to convert symbol coding to binary code groups. If these errors are too frequent in nature, the error correction capabilities of the DTV receiver are overwhelmed, and there is catastrophic failure in the television image. If such catastrophic failure occurs infrequently, it can be masked to some extent by freezing the last transmitted good TV images, such masking being less satisfactory if the TV images contain considerable motion content. The catastrophic failure in the television image is accompanied by loss of sound.
The training signal or ghost-cancellation reference (GCR) signal in the initial line of each data field of an ATSC-standard DTV signal is a 511-sample pseudo-random noise sequence (or "PN sequence") followed by three 63-sample PN sequences. A 511-sample PN sequence is referred to as a "PN511 sequence" and a 63-sample PN sequence is referred to as a "PN63 sequence". The middle ones of the 63-sample PN sequences in the field synchronization codes are transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. This training signal has not worked well in practice, however, and cannot be incorporated in its entirety into an NTSC television signal.
The middle PN63 sequence of the ATSC field synchronization code, as separated by differentially combining corresponding samples of successive field synchronization code sequences, can be used as a basis for detecting ghosts. Pre-ghosts of up to 53.701 microseconds (4+511+63=578 symbol epochs) before the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the last data segment of the preceding data field. However, the post-ghosts of such data can extend up to forty microseconds into the first data segments and add to the background clutter that has to be discriminated against when detecting pre-ghosts of the separated middle PN63 sequence. Post-ghosts of up to 17.746 microseconds (63+104+24=191 symbol epochs) after the separated middle PN63 sequence can be detected in a discrete Fourier transform (DFT) procedure without have to discriminate against data in the precode and in the data segment of the succeeding data field. Longer-delayed post-ghosts have to be detected while discriminating against background clutter that includes data. The autocorrelation properties of the PN63 sequence are not so great that detection of longer-delayed post-ghosts may be sufficiently sensitive. The middle PN63 sequence of the ATSC field synchronization code provides more pre-ghost canceling capability than required in practice, but insufficient post-ghost canceling capability. Post-ghosts delayed up to forty microseconds after principal signal occur in actual practice. However, pre-ghosts preceding the principal signal by more than four microseconds are rare, according to page 3 of the T3S5 Report Ghost Canceling Reference Signals published Mar. 20, 1992 by the ATSC.
If one seeks to exploit the autocorrelation properties of the PN511 sequence in the ATSC DTV signal for selection of ghosts in a DFT procedure, the selection filter has to discriminate PN511 sequence and its ghosts from background clutter that includes data and the initial and final PN63 sequences. This background clutter has substantial energy, so weaker ghosts of the PN511 sequence are difficult to detect. The higher energy response of the PN511 autocorrelation filter used for ghost detection cannot be fully exploited because data and the initial and final PN63 sequences increase so much the energy of the background clutter that the filter is to discriminate against.
The current de facto standard for ghost-cancellation reference (GCR) signal in an analog television signal transmitted in accordance with the National Television System Committee (NTSC) standard is as follows. A Bessel chirp is transmitted in the nineteenth vertical-blanking-interval (VBI) horizontal scan line of each field. The cost of ghost-cancellation circuitry is quite high, somewhat over $100 in the retail price of a receiver, so few analog TV receivers with ghost-cancellation circuitry have been commercially manufactured. The inventors believe that television receivers capable of receiving either DTV or NTSC signals, referred to in this document as "NTSC/DTV receivers", will be the norm during a period of transition from NTSC TV broadcasting to DTV broadcasting. Ghost-cancellation and equalization circuitry is a practical necessity in the DTV portion of the TV receiver. Accordingly, the inventors point out, it can be economical to use at least part of that same ghost-cancellation and equalization circuitry to suppress ghosts in the NTSC portion of the TV receiver.
This dual usage of the same ghost-cancellation and equalization circuitry is furthered by the nineteenth VBI scan line of each field including a GCR signal similar to that used in the DTV signal rather than the Bessel chirp that is the current standard. The use of a similar GCR signal during DTV transmission and during NTSC transmission, rather than using different GCR signals, expedites using the same microcomputer program to calculate weighting coefficients for the ghost-cancellation and equalization filters during the reception of each type of transmission. The desirability of using a similar GCR signal during DTV transmission and during NTSC transmission, in order to reduce hardware in an NTSC/DTV receiver, has not been previously recognized, insofar as the inventors are aware.
The inventors observe that the 10.76.multidot.10.sup.6 baud rate of DTV using the ATSC standard and the 3.58 MHz color subcarrier frequency of NTSC TV have harmonics that are close in frequency, facilitating the construction of a sampling clock generator for the digital filtering used in the ghost-cancellation and equalization circuitry, which sampling clock generator is susceptible of receiving automatic frequency and phase control (AFPC) signal either from the 3.58 MHz color subcarrier frequency regenerated during NTSC TV reception or from the baud rate information extracted during DTV reception.